The present invention relates to a capacitor element for a power capacitor comprising a first foil of metal material and a second foil of metal material arranged to form a pair of electrodes, and also films of dielectric material arranged between the foils in order to form a dielectric medium, which capacitor element is produced by said foils and films being arranged one on top of the other and wound to a roll, the capacitor element displaying a first end surface where the second foil has a long edge indented between two of said films situated adjacent to the second foil, and also a second end surface where the first foil has a long edge indented between two of said films situated adjacent to the first foil, each of which indented long edges has an edge portion surrounded by an elongate field equalizer of dielectric material surrounding the edge portion.
The invention also relates to a power capacitor comprising such a capacitor element.
In this context xe2x80x9cpower capacitorxe2x80x9d refers to a capacitor for alternating or direct current applications for voltages exceeding 1 kV.
A capacitor element of the type mentioned above is often referred to as a wound or wrapped capacitor element of film-foil type. The films and foils are in the form of elongate, thin, substantially rectangular strips with parallel long edges. When manufacturing capacitor elements the films and foils are arranged one on top of the other and wound to said roll form, so that in its radial direction the capacitor displays several layers of foils with intermediate layers of films. The foils thus form the electrodes of the capacitor element and the films form the dielectric medium of the capacitor element. To prevent electric flashover between the electrodes at the end surfaces of the capacitor element one foil, the first, is displaced in axial direction of the capacitor element towards one end surface, the first, of the capacitor element, so that the first foil displays a long edge that is indented between the films at the second end surface of the capacitor element. Similarly, the second foil is displaced in axial direction of the capacitor element towards the second end surface of the capacitor element so that the second foil displays a long edge indented between the films at the first end surface of the capacitor element. Since a certain indentation is necessary to ensure electric insulation between the electrodes, the indentation entails a limitation of the area the electrodes connect capacitively to each other, which is not desirable.
When constructing a power capacitor a plurality of capacitor elements of the type described above are connected together in series and in parallel. The connected capacitor elements form a capacitor stack which is enclosed in a container and connected electrically by means of connection contacts which, with the aid of bushings, pass through the container. In order to save space it is usual for the roll-shaped capacitor elements to be flattened to non-circular cross-section before they are connected together. The capacitor elements are usually impregnated in the container with an impregnating agent, such as some form of oil.
During operation the electrodes in each capacitor element are placed under voltage, whereupon a voltage difference, termed xe2x80x9celement voltagexe2x80x9d, is produced between the electrodes. Due to the element voltage an electric field is produced in the dielectric medium between the electrodes, having an electric field intensity that is substantially proportional to the element voltage and inversely proportional to the distance between the electrodes, i.e. to the thickness of the dielectric medium. A rated or average field intensity, Emean=U/d is calculated for each capacitor element, where U is the element voltage and d the thickness of the dielectric medium. This average field intensity gives an approximate value of the field intensity in the dielectric medium inside the indented long edges of the electrodes. As high an average field intensity as possible is aimed at in a capacitor element. For instance the reactive power produced by a capacitor element for alternating current applications is substantially proportional to the average field intensity squared. However, field concentrations occur at the indented long edges where the field intensity locally assumes values that greatly exceed the average field intensity. If steps are taken to increase the average field intensity, e.g. if the element voltage is increased, the field intensity at the indented long edges will also increase. If the average field intensity exceeds a critical limit value, so-called partial discharges will occur at the indented long edges of the foils, due to the field concentrations. Partial discharges, also termed glow discharges, are electrical discharges that break down the dielectric medium. In time partial discharges may cause penetration of the insulation in the electric medium, resulting in short-circuiting between the electrodes.
One way of reducing field concentrations at the long edges of a capacitor element is described in U.S. Pat. No. 2,528,596 A. The capacitor element is provided with narrow dielectric strips that are folded around the indented long edges for the purpose of increasing the thickness of the dielectric medium locally around these long edges, thereby improving the electrical strength.
Another way of reducing the field concentrations at the long edges of a capacitor element is described in U.S. Pat. No. 4,320,437 A where the electrode, either in its entirety or just its long edges, is coated with a dielectric material to form a very thin coating having considerable dielectric constant. The purpose of the coating is not to increase the thickness of the dielectric medium but to replace the impregnation liquid nearest the electrode. The dielectric constant of the coating shall be as great as or greater than the dielectric constant of the impregnation liquid. A number of coating techniques are mentioned, all of which are more or less technically complicated and produce an extremely thin coating. The preferred coating thickness is stated to be in the interval 0.12-0.25 xcexcm, and a thickness exceeding 5 xcexcm is said to be unusable.
Yet another known method of reducing field concentrations is to shape the indented long edges of the foils as gently as possible, i.e. with as few sharp edges as possible. It is known, for instance, to use foils with laser-cut long edges, so that rounded long edges, without sharp edges are obtained. It is also known to fold the indented long edges double, thereby giving long edges with gentle semi-circular shape. Despite these measures, for many power capacitor applications partial discharges at the indented long edges limit the average field intensity that can be permitted in a conventional capacitor element.
The object of the present invention is to provide a new capacitor element of the type described in the introduction, which effectively equalizes the increased electrical strength resulting from said field equalizers.
The capacitor element and the power capacitor in accordance with the invention are characterized in that the field equalizer is in the form of an elongate strip having a thickness within the interval 5-10 xcexcm which is folded around the indented long edge and surrounds the latter and which extends axially through the capacitor element.
Thanks to their thickness and to the field equalizers surrounding the indented long edges and extending through the capacitor element, the field equalizers contribute not only to the electrical strength increasing at the indented long edges but also to the thickness of the dielectric medium increasing. The element voltage that can be connected between the electrodes is thus increased and fewer capacitor elements therefore need to be series-connected in a power capacitor in accordance with the invention in order to achieve a predetermined voltage over the connection contacts of the power capacitor. Since thicker dielectric medium means that the volume taken up in the capacitor element decreases and the volume taken up by the dielectric medium increases, the energy density in the capacitor element will also be higher. In one embodiment the field equalizers consist of the same dielectric material as the dielectric medium of the capacitor element. This offers the advantage of fewer types of material having to be used when manufacturing the capacitor element, and thus simpler handling at the manufacturing stage.